Why do bathroom mirrors fog up?
Short answer
Water vapor in the warm bathroom air turns into tiny liquid droplets on the relatively cool mirror. Those droplets scatter light, making the mirror look white and foggy.
How it works
A steamy bathroom contains a lot of water vapor. When the warm, moist air touches the mirror, the mirror cools the nearby water vapor.
When water vapor changes from a gas into liquid water, the process is called condensation. The very small droplets on the mirror scatter reflected light in many directions, so the image becomes difficult to see.
Key point
Warming the mirror or ventilating the room can make droplets less likely to form. Glasses and windows fogging after someone breathes on them are examples of the same condensation process.